31 research outputs found
Sedimentation and Levitation of Catalytic Active Colloids
Gravitational effects in colloidal suspensions can be easily turned off by
matching the density of the solid microparticles with the one of the
surrounding fluid. By studying the motion of catalytic microswimmers with
tunable buoyant weight, we show that this strategy cannot be adopted for active
colloidal suspensions. If the average buoyant weight decreases, pronounced
accumulation at the top wall of a sample cell is observed due to a
counter-alignment of the swimming velocity with the gravitational field. Even
when the particles reach a flat wall, gravitational torques still determine the
properties of the quasi two-dimensional active motion. Our results highlight
the subtle role of gravity in active systems.Comment: 4 pages, 4 figure
Microswimmers in Patterned Environments
We demonstrate with experiments and simulations how microscopic
self-propelled particles navigate through environments presenting complex
spatial features, which mimic the conditions inside cells, living organisms and
future lab-on-a-chip devices. In particular, we show that, in the presence of
periodic obstacles, microswimmers can steer even perpendicularly to an applied
force. Since such behaviour is very sensitive to the details of their specific
swimming style, it can be employed to develop advanced sorting, classification
and dialysis techniques.Comment: 4 pages, 4 figure
Dynamical clustering and phase separation in suspensions of self-propelled colloidal particles
We study experimentally and numerically a (quasi) two dimensional colloidal
suspension of self-propelled spherical particles. The particles are
carbon-coated Janus particles, which are propelled due to diffusiophoresis in a
near-critical water-lutidine mixture. At low densities, we find that the
driving stabilizes small clusters. At higher densities, the suspension
undergoes a phase separation into large clusters and a dilute gas phase. The
same qualitative behavior is observed in simulations of a minimal model for
repulsive self-propelled particles lacking any alignment interactions. The
observed behavior is rationalized in terms of a dynamical instability due to
the self-trapping of self-propelled particles.Comment: 8 pages including supplemental information, to appear in Phys. Rev.
Let
Reply to Comment on "Circular Motion of Asymmetric Self-Propelling Particles"
In a Comment [Phys. Rev. Lett. 113, 029801 (2014)] on our Letter on
self-propelled asymmetric particles [Phys. Rev. Lett. 110, 198302 (2013);
arXiv:1302.5787], Felderhof claims that our theory based on Langevin equations
would be conceptually wrong. In this Reply we show that our theory is
appropriate, consistent, and physically justified.Comment: 2 page
Island Hopping of active colloids
Individual self-propelled colloidal particles, like active Brownian particles
(ABP) or run-and-tumble swimmers (RT), exhibit characteristic and well-known
motion patterns. However, their interaction with obstacles remains an open and
important problem. We here investigate the two-dimensional motion of
silica-gold Janus particles (JP) actuated by AC electric fields and suspended
and cruising through silica particles organized in rafts by mutual phoretic
attraction. A typical island contains dozens of particles. The JP travels
straight in obstacle-free regions and reorients systematically upon approaching
an island. As an underlying mechanism, we tentatively propose a hydrodynamic
torque exerted by the solvent flow towards the islands on the JP local flow
field, leading to an alignment of respective solvent flow directions. This
systematic behavior is in contrast with the reorientation observed for free
active Brownian particles and run-and-tumble microswimmers.Comment: 13 pages,4 main figures, 2 supplementary figure
Active Brownian Motion Tunable by Light
Active Brownian particles are capable of taking up energy from their
environment and converting it into directed motion; examples range from
chemotactic cells and bacteria to artificial micro-swimmers. We have recently
demonstrated that Janus particles, i.e. gold-capped colloidal spheres,
suspended in a critical binary liquid mixture perform active Brownian motion
when illuminated by light. In this article, we investigate in some more details
their swimming mechanism leading to active Brownian motion. We show that the
illumination-borne heating induces a local asymmetric demixing of the binary
mixture generating a spatial chemical concentration gradient, which is
responsible for the particle's self-diffusiophoretic motion. We study this
effect as a function of the functionalization of the gold cap, the particle
size and the illumination intensity: the functionalization determines what
component of the binary mixture is preferentially adsorbed at the cap and the
swimming direction (towards or away from the cap); the particle size determines
the rotational diffusion and, therefore, the random reorientation of the
particle; and the intensity tunes the strength of the heating and, therefore,
of the motion. Finally, we harness this dependence of the swimming strength on
the illumination intensity to investigate the behaviour of a micro-swimmer in a
spatial light gradient, where its swimming properties are space-dependent
Feedback-Controlled Active Brownian Colloids with Space-Dependent Rotational Dynamics
The non-thermal nature of self-propelling colloids offers new insights into
non-equilibrium physics. The central mathematical model to describe their
trajectories is active Brownian motion, where a particle moves with a constant
speed, while randomly changing direction due to rotational diffusion. While
several feedback strategies exist to achieve position-dependent velocity, the
possibility of spatial and temporal control over rotational diffusion, which is
inherently dictated by thermal fluctuations, remains untapped. Here, we
decouple rotational diffusion from thermal noise. Using external magnetic
fields and discrete-time feedback loops, we tune the rotational diffusivity of
active colloids above and below its thermal value at will and explore a rich
range of phenomena including anomalous diffusion, directed transport, and
localization. These findings add a new dimension to the control of active
matter, with implications for a broad range of disciplines, from optimal
transport to smart materials